CNOOC is one of China's three major state-owned oil and gas producers. It bought Canada's Nexen last year in a $15 billion deal.

The Beijing-based company said oil and gas sales grew last year because of "stable growth" in production. But it suffered from higher expenses on exploring for new wells as well as higher costs to operate existing wells.

The company also said foreign operating expenses jumped 25 percent because of increased oil sands production.